A string of earthquakes near the vulnerable San Francisco Bay Area erupted in rapid succession Monday morning, even triggering California’s tsunami alert system.
The earthquake swarm started with a magnitude 3.8 quake at 9.27am ET, with the US Geological Survey (USGS) recording ten sizeable seismic events in just 40 minutes around San Ramon.
Within two hours, USGS recorded more than 19 earthquakes stronger than magnitude 2.5, meaning they were all powerful enough for people to feel above ground.
A magnitude 4.2 earthquake was detected at 10.01am ET, which sent noticeable shockwaves to nearby San Francisco, Sacramento, and San Jose.
Residents in San Francisco’s Glen Park and Nopa neighborhoods reported rumbling and ‘windows rattling’ during the quake, and public transportation throughout the area was also affected by the swarm, according to the San Francisco Chronicle.
The moderate quake even activated the National Tsunami Warning Center, however, officials said there was no danger of a major wave hitting the Bay Area.
The city of San Ramon is less than 40 miles from San Francisco, and less than 30 miles from major cities such as Oakland, Concord, and San Jose.
San Ramon lies atop the Calaveras Fault, a major branch fault in the broader San Andreas Fault System, sparking fears the main fault line could be closer to a major rupture that locals refer to as ‘the Big One.’
The initial magnitude 3.8 quake was centered just 16 miles from Concord, 18 miles from Oakland, and 29 miles from San Jose, which are three of the most populated communities in the Bay Area, home to more than 1.5 million people.
Meanwhile, San Francisco, which sits just across the bay from where the seismic swarm took place, has a population of more than 800,000 residents.
Despite considerable shaking throughout the Bay Area, there have been no reports of injuries or property damage at this time.
Still, Monday’s noticeable swarm broke out right on the Calaveras Fault, a main branch of the San Andreas – the monstrous 800-mile-long fault spanning from Southern California through the Bay Area and into the Pacific.
The Calaveras splits off from the main fault line near Hollister in central California and runs parallel to it through the East Bay region.
Scientists with USGS have warned that one of these faults or other major branches nearby could soon reach their anticipated breaking point and rupture right in the heart of California.
According to a 2015 report by USGS scientists, there is a 95 percent probability that at least one major quake, stronger than magnitude 6.7, will strike somewhere in the region by 2043.
A magnitude 6.7 earthquake on the Calaveras Fault would be classified as a major seismic event capable of causing significant damage in densely populated East Bay communities.
Scientists are still working to confirm which fault line produced the new string of earthquakes.
Meanwhile, USGS has admitted that it’s still unclear why earthquake ‘swarms,’ a rapid and ongoing series of tremors, even take place underground, noting that’s there’s a missing component causing it that hasn’t been discovered.
A statement from USGS said: ‘Based on past research, we understand that swarms probably indicate that an extra “ingredient” is involved where the earthquakes are happening – an ingredient that isn’t as prevalent in mainshock-aftershock sequences.’
The leading theory at this time is that earthquake swarms are often triggered by underground water or other fluids flowing into nearby faults.
USGS has said that high-pressure water deep in the Earth likely moves along cracks in the rock and pushes against the sides of a fault. This makes it easier for the fault to slip and cause small earthquakes.
When those earthquakes occur, they open up or widen tiny pathways in the rock. This lets even more water flow in and spread out. The extra water then puts pressure on nearby parts of the fault, triggering more small quakes.
